1. Field of the Invention
This invention relates to ion implantation apparatus to implant ions into planar workpieces. Specific applications of the ion implantation apparatus include the production of lamina of crystalline semiconductor material, such as silicon. Such silicon laminae may be used for the production of photovoltaic cells.
2. Background information
As the demand for renewable energy based on renewable sources increases, the implementation of photovoltaic technology has expanded dramatically in recent years. Nevertheless, a way of forming crystalline silicon bodies optimized for photovoltaic cells has remained elusive.
Crystalline silicon wafers adapted to bear photovoltaic cells are conventionally obtained by slicing a silicon ingot. This process, which typically yields a silicon wafer thicker than 150 μm, wastes a substantial amount of silicon by consuming up to 50% of the silicon body in kerf loss and delivering a much greater thickness than is needed for useful photovoltaic devices.
Thinner silicon laminae have been made by exfoliation of a film by heating after high-dose ion implantation. The films produced this way have found application in forming silicon-on-insulator structures but have been cost-prohibitive for solar cells. Also at thickness well under 1 μm, the films may be so thin as to make efficient light-capturing difficult. Boosting the energy of ion implant could increase the film thickness, but this adaptation would make the films even more expensive and less economical for photovoltaic cells.
There is accordingly, a need for a cost-effective way to form silicon bodies optimized for photovoltaic applications.
A known type of ion implantation tool has an ion source which produces a beam containing ions to be implanted. The ion beam is directed through a region of homogeneous magnetic field in an ion filter to provide spatial separation between ions in the beam with different momentum over charge (mv/e) ratios. A mass selector slit blocks any unwanted ions and allows desired ions to pass, optionally through an electrostatic accelerator, to a process chamber for implantation in semiconductor substrates or wafers. To improve productivity, a batch of wafers may be processed simultaneously by mounting them round the periphery of a process wheel mounted for rotation about an axis, so that the wafers on the wheel pass one after the other through the ion beam. The process wheel axis is at the same time translated towards and away from the beam to provide a two dimensional mechanical scan of the wafers through the ion beam, to ensure all parts of the wafers are implanted, even though the ion beam may have a cross sectional area as it strikes the wafers which is smaller than the wafer area.
One known batch implanter, which is a variant of the above general type, has a large process wheel with a fixed vertical axis and a radially scanned ion beam.
A further known type of implantation tool produces a so-called ribbon beam of ions, having a major dimension sufficient to extend right across a single wafer. A ribbon beam arrangement of this kind requires the wafers to be mechanically scanned only in one dimension, transverse to the ribbon beam plane. This is usually accomplished with a translational scanning holder carrying a single wafer, so that wafers are implanted serially one at a time. A magnetic mass selecting ion filter is used to bend the ribbon beam transversely to the plane of the ribbon beam, so that desired ions from the ribbon beam can be selected by a relatively narrow slit extending parallel to the ribbon beam plane. Alternatively, if the ion beam is bent in the plane of the ribbon, the ribbon is brought to a focus in the x-direction (the ribbon plane), to pass through a narrow mass selection slit, before being expanded again and collimated into a ribbon beam.